The present invention relates to means for removing from upright, tilt-up building panels the lifting devices used for erecting the panel without requiring a workman to climb to the height at which the lifting device is attached to the panel.
Tilt-up building panels or slabs are increasingly used in the construction of large buildings, usually commercial or industrial buildings. In such constructions concrete panels are formed on the floor of the building or on a level ground surface, they are then lifted to a vertical or upright position and interconnected to form the walls of the building.
The lifting device for each panel must be removed after the panel is in place. Since the lifting device is normally attached at a substantial height, the panels ranging in height to up to thirty feet or more, it was heretofore necessary that a workman position a ladder against a panel and that he climb up a ladder so that he can reach the lifting device and disconnect it. To facilitate this task, a quick-release connector has heretofore been devised for coupling the hoisting device, e.g., the crane hook to the panel. This connector is described and claimed in U.S. Pat. No. 3,456,547.
Briefly, the quick release connector described in that patent comprises an insert that is cast into the panel and which comprises an internally threaded nut the opening of which is flush with the side of the panel. The threaded opening is engaged with a split bolt formed of a pair of separate longitudinal bolt segments which can be brought together or collapsed sufficiently so that the segments can be axially inserted into the threaded opening of the insert without contacting the insert. Thus, the segments can be linearly inserted and need not be threaded into the insert. After the bolt segments are inserted a wedge or spreader is placed between them to expand them into engagement with internal threads of the insert. This expansion locks the split bolt to the insert. To remove the split bolt the wedge is axially withdrawn and the two bolt segments are brought together again to disengage them from the insert. Thereafter the bolt segments can be axially withdrawn from the opening.
When the split bolt is inserted, a portion of its shank protrudes past the insert and engages a collar or a similar structural member which includes an eyelet, a ring or the like to which the crane hook or similar panel lifting device is releasably attached.
Although the above described split bolt for releasably securing the lifting device to building panels is a great improvement over the prior art and substantially reduces the time required for attaching and releasing the lifting device from the panel, it nevertheless requires that a workman place a ladder against the upright panel to reach and release the split bolt. Since building panels are large and heavy, each panel requires several spaced apart lifting devices. This in turn requires the workmen to reposition the ladder and climb up for the release of each of the split bolts. While he performs this work, the crane or other hoisting equipment, which normally comprises the most expensive (per hour) piece of equipment on a construction job, is idle. Even though only relatively little time may be required to loosen any one split bolt the total time is significant, because there are often hundreds or thousands of inserts on large construction sites from which the lifting devices must be removed. When multiplied by the hourly rate for a crane and its operator, the additional costs are substantial; they often amount to thousands of dollars per site.
In addition, the necessity for placing ladders against upright, tilt-up panels and the need for climbing them represents a safety hazard. Building sites have a traditionally uneven ground which is often cluttered with debris and which is sometimes relatively soft. Thus, unless the ladder is positioned with care and skill its footing might be unstable and might give way under the weight of the workman thereon. At the often substantial heights of twenty, thirty or more feet of such panels, this is dangerous. Moreover, there is the further danger that the temporarily braced panel is insufficiently supported and may fall under the lateral force applied by the ladder positioned thereagainst due to an improperly positioned temporary panel support. This can be fatal not only to the workman on the ladder but to other workmen in the vicinity of the falling panel.